Clock generators are provided in individual components of digital communications networks such as, for example, switching and transmission equipment for the time-related switching and transmission of the digital information. The clock signals required for the time-related control of the respective equipment are generated in these clock generators.
For plesiochronic operation of the communications network, clock signals that are extremely accurate in bit rate and that are stable over a long term must be formed in the individual clock generators. Although this operating mode requires an extremely high outlay per clock generator, it permits extremely flexible and wide-ranging network formatting.
A highly accurate, outage-protected clock generator is centrally installed in synchronously operated communications networks. The clock signals generated in this clock generator are communicated to all components of the communications network via transmission-oriented equipment together with digital information, for example, voice information. Respective clock signals whose phases coincide with the phases of incoming, highly accurate clock signals are generated in the components using clock generator equipment equipped with phase-locked loops. Such a clock generator having a phase-locked loop is known from the periodical "Proceedings of 1979 ISCAS", Pages 804, 805 "A Microprocessor-Controlled Phase-Locked Loop For Network Synchronization". A time-related handling of the digital information is guaranteed in the individual network components by these clock generators. Compared to plesiochronic operation of a communications network, the outlay per clock generator in view of clock precision and, in particular, long-term stability is lower for synchronous operation; however, this involves a more complicated transmission technology and a spatially limited network structure.
Outage-protected clock generators are provided in network nodes, particularly digital switching equipment, for both plesiochronic operation and synchronous operation of a communications network. To this end, two identically structured clock generators, i.e. clock generators equipped with phase-locked loops, are provided, whereby the clock signals of one clock generator (also referred to hereinafter as a first reference clock generator which provides "first reference clock signals") are synchronized to the incoming, highly accurate clock signals and the clock signals of another clock generator (also referred to hereinafter as a second clock generator which provides "second clock signals") are synchronized to the reference clock signals formed by the reference clock generator. An automatic switch to the second clock generator is usually undertaken for outage of the first reference clock generator. Since both clock generators are located, for example, in switching equipment which is frequently spaced some distance from one another, for example, in different cabinet racks, the phase relations of the clock signals generated in the two clock generators deviate in communications networks having high processing speeds, that is, clock signals with high bit rates, due to signal running times via connections between the clock generators. Given an outage of the first reference clock generator, these phase deviations cause considerable disturbances after switching to the second clock generators, particularly for digital information switching and transmission. Particularly in digital data transmission, this can lead to transmission delays and, under certain conditions, to abnormal connection termination.